Modern radio frequency (RF) systems generally require highly linear RF power amplifiers in order to properly transmit signals. To provide high linearity, RF power from an RF power amplifier must remain constant over the entirety of a data frame despite changes in duty cycle, ambient temperature, and supply voltage. Maintaining a constant RF power generally requires a tightly controlled bias signal to be provided to the RF power amplifier, which may be difficult due to the dynamic signal environments in which modern RF systems exist. Maintaining tight control over the bias signal will generally reduce the error vector magnitude and other metrics of linearity such as amplitude modulation AM to AM distortion of the RF power amplifier and therefore allow the amplifier to meet the stringent specifications of modern wireless communications standards such as WiFi.
A power amplifier voltage regulator is generally responsible for providing a bias signal to a power amplifier. As discussed above, it is desirable for a power amplifier voltage regulator to provide a highly stable bias signal that is not affected by process variation of one or more components therein. Further, it is desirable for a power amplifier voltage regulator to maintain a temperature slope that is suited for the bias circuits of the power amplifier with which it is used, achieve very short turn-on time such that the power amplifier is able to accurately amplify preambles of data frames, have a small size, and consume a low quiescent current to reduce battery drain (e.g., less than 0.5 μA). Generally, it is very difficult to design a power amplifier voltage regulator that meets all of these criteria.
FIG. 1 is a functional schematic illustrating an idealized power amplifier voltage regulator 10. The idealized power amplifier voltage regulator 10 includes an operational amplifier 12, a first feedback resistor RF, and a second feedback resistor RG. The operational amplifier 12 includes a power supply voltage node NVPS, a non-inverting input node NNI, an inverting input node N1, and an output node NREG. A power supply voltage VPS is provided to the power supply voltage node NVPS. An input voltage VIN is provided to the non-inverting input node NNI, and may be equal to the power supply voltage VPS. A regulated output voltage VREG is provided at the output node NREG. The inverting input node NI is coupled to the junction between the first feedback resistor RF and the second feedback resistor RG and thus receives a feedback signal therefrom. Given the feedback structure of the idealized power amplifier voltage regulator 10, the gain of the device can be expressed by Equation (1):
                    G        =                                            V              REG                                      V              IN                                =                      1            +                          (                                                R                  F                                                  R                  G                                            )                                                          (        1        )            Those skilled in the art will appreciate that the operational amplifier 12 will attempt to equalize the voltage at the non-inverting input node NNI and the inverting input node NI via the feedback formed between the output node NREG and the inverting input node NI. The feedback provided at the inverting input node NI effectively ensures that the regulated output voltage VREG is tightly controlled regardless of process variations inherent in the operational amplifier 12 and tracks ambient temperature. If the operational amplifier 12 acts ideally, then it will also provide short turn-on times and relatively low quiescent current consumption.
Implementing the idealized voltage regulator circuitry in a mobile device is a challenging task. Generally, standard operational amplifiers are created via CMOS processes and are too large for mobile applications. Accordingly, there is a need for voltage regulator circuitry for a mobile device with a small footprint that provides a tightly controlled output voltage that tracks ambient temperature, short turn-on times, and low quiescent current consumption.